Effects of Heat Acclimation and Acclimatisation on Maximal Aerobic Capacity Compared to Exercise Alone in Both Thermoneutral and Hot Environments: A Meta-Analysis and Meta-Regression

Abstract

Background Heat acclimation and acclimatisation (HA) is typically used to enhance tolerance to the heat, thereby improving performance. HA might also confer a positive adaptation to maximal oxygen consumption ($$V{\text{O}}_{2\max }$$ V O 2 max ), although this has been historically debated and requires clarification via meta-analysis. Objectives (1) To meta-analyse all studies (with and without control groups) that have investigated the effect of HA on $$V{\text{O}}_{2\max }$$ V O 2 max adaptation in thermoneutral or hot environments; (2) Conduct meta-regressions to establish the moderating effect of selected variables on $$V{\text{O}}_{2\max }$$ V O 2 max adaptation following HA. Methods A search was performed using various databases in May 2020. The studies were screened using search criteria for eligibility. Twenty-eight peer-reviewed articles were identified for inclusion across four separate meta-analyses: (1) Thermoneutral $$V{\text{O}}_{2\max }$$ V O 2 max within-participants (pre-to-post HA); (2) Hot $$V{\text{O}}_{2\max }$$ V O 2 max within-participants (pre-to-post HA); (3) Thermoneutral $$V{\text{O}}_{2\max }$$ V O 2 max measurement; HA vs. control groups; (4) Hot $$V{\text{O}}_{2\max }$$ V O 2 max measurement, HA vs. control groups. Meta-regressions were performed for each meta-analysis based on: isothermal vs. iso-intensity programmes, days of heat exposure, HA ambient temperature (°C), heat index, HA session duration (min), ambient thermal load (HA session x ambient temperature), mean mechanical intensity (W) and the post-HA testing period (days). Results The meta-analysis of pre–post differences in thermoneutral $$V{\text{O}}_{2\max }$$ V O 2 max demonstrated small-to-moderate improvements in $$V{\text{O}}_{2\max }$$ V O 2 max (Hedges’ g = 0.42, 95% CI 0.24–0.59, P < 0.001), whereas moderate improvements were found for the equivalent analysis of hot $$V{\text{O}}_{2\max }$$ V O 2 max changes (Hedges’ g = 0.63, 95% CI 0.26–1.00, P < 0.001), which were positively moderated by the number of days post-testing (P = 0.033, β = 0.172). Meta-analysis of control vs. HA thermoneutral $$V{\text{O}}_{2\max }$$ V O 2 max demonstrated a small improvement in $$V{\text{O}}_{2\max }$$ V O 2 max in HA compared to control (Hedges’ g = 0.30, 95% CI 0.06–0.54, P = 0.014) and this effect was larger for the equivalent hot $$V{\text{O}}_{2\max }$$ V O 2 max analysis where a higher (moderate-to-large) improvement in $$V{\text{O}}_{2\max }$$ V O 2 max was found (Hedges’ g = 0.75, 95% CI 0.22–1.27, P = 0.005), with the number of HA days (P = 0.018; β = 0.291) and the ambient temperature during HA (P = 0.003; β = 0.650) positively moderating this effect. Conclusion HA can enhance $$V{\text{O}}_{2\max }$$ V O 2 max adaptation in thermoneutral or hot environments, with or without control group consideration, by at least a small and up to a moderate–large amount, with the larger improvements occurring in the heat. Ambient heat, number of induction days and post-testing days can explain some of the changes in hot $$V{\text{O}}_{2\max }$$ V O 2 max adaptation.

Heat suit training increases hemoglobin mass in elite cross‐country skiers

“…Together, these aspects points to the limited statistical power of the present study to detect changes in cardio‐respiratory parameters and subsequent performance. The observed lack of statistical effects of heat training on V̇O 2max despite an increase in Hb mass is in agreement with previous studies investigating the effects of prolonged heat training, 6,34 though a recent meta‐analysis revealed that 5–60 days of heat training can enhance V̇O 2max in thermoneutral environments by a magnitude likely missed in small scale studies 35 . Furthermore, there were no differences between the groups in any of the other investigated physiological determinants of endurance performance such as work economy, fractional utilization of V̇O 2max , running velocity associated with 4 mmol·L −1 [La − ], or time to exhaustion during the graded V̇O 2max test to exhaustion.…”

“…The observed lack of statistical effects of heat training on VȮ 2max despite an increase in Hb mass is in agreement with previous studies investigating the effects of prolonged heat training, 6,34 though a recent meta-analysis revealed that 5-60 days of heat training can enhance VȮ 2max in thermoneutral environments by a magnitude likely missed in small scale studies. 35 Furthermore, there were no differences between the groups in any of the other investigated physiological determinants of endurance performance such as work economy, fractional utilization of VȮ 2max , running velocity associated with 4 mmol•L −1 [La − ], or time to exhaustion during the graded VȮ 2max test to exhaustion. The present findings are partly in agreement with previous 5 weeks heat training studies where a 3-4.5% increase in Hb mass failed to induce a significant performance increase.…”

Heat Training Efficiently Increases and Maintains Hemoglobin Mass and Temperate Endurance Performance in Elite Cyclists

“…This may seem surprising, as there is a strong relationship between total Hb mass and V̇O 2max , with each g increase in hemoglobin having been proposed to lead to 4 mL·min −1 increases in V̇O 2max (31). Regardless of this, the observed lack of statistical effects of heat training on V̇O 2max despite an increase in Hb mass is in agreement with previous studies investigating the effects of prolonged heat training (4,8), although a recent meta-analysis revealed that heat training can enhance V̇O 2max in thermoneutral environments by a small, but significant amount (32). Furthermore, V̇O 2max is influenced by factors other than Hb mass (1).…”

Heat acclimation does not attenuate hepcidin elevation after a single session of endurance exercise under hot condition

“…The present findings suggest that short-term HA training under hot conditions does not affect iron metabolism, at least in terms of hepcidin activity. Therefore, endurance training under hot conditions can improve thermoregulatory capacity and endurance capacity (Lorenzo et al 2010 ; Tyler et al 2016 ; Saunders et al 2019 ; Waldron et al 2021 ) without affecting iron metabolism.…”

“…However, the effects of HA on exercise-induced hepcidin elevations remains unclear. Endurance training under hot conditions (i.e., HA training) is a well-established method to improve thermoregulatory capacity and endurance performance (Lorenzo et al 2010 ; Tyler et al 2016 ; Saunders et al 2019 ; Waldron et al 2021 ), and is widely accepted as a useful form of training among various types of endurance athletes. A better understanding of hepcidin activity in association with iron metabolism is important, as endurance athletes commonly incorporate several days of endurance training under hot conditions into their training schedule (i.e., training camps).…”